The audible range of many hearing impaired individuals is compressed to a limited dynamic range of sound. For such individuals, soft sounds may be inaudible while loud sounds are heard at the same sound level as persons with normal hearing. Hearing aids are electronic devices which are often used to help individuals with hearing impairment lead a relatively normal life. Such hearing aids sometimes employ automatic gain control (AGC) circuitry to compensate for these hearing deficiencies. Such circuits are often designed to compress the sound level delivered to their users by providing greater amplifier gain for soft sounds and reduced gain for loud sounds.
AGC circuits are used because traditional amplifiers many times are unable to accurately reproduce an input signal because of limitations in the amplifier performance capabilities. Consequently, there may be magnitudes or values of the input signal that cannot be amplified properly. In such situations, a significant amount of distortion is generated at the amplifier output, and the output signal is “clipped” when the peak values of that signal are forced by the amplifier to not exceed some internally determined clipping limit.
AGC circuits are therefore employed to compress the output signal by reducing the gain of the amplifier whenever the output signal becomes too great. Such a circuit arrangement advantageously avoids much of the distortion that occurs due to clipping. The rapidity at which an AGC circuit reacts to the occurrence of unusually large signal magnitudes at the amplifier output, both the time duration in which gain changes are initiated and the duration in which gain changes are terminated, can cause different effects in the subsequent signal portions. Quick implementation of gain changes allows affecting the spoken syllables in an audio signal while slow implementation of gain changes allows the system to respond to the sound background to thereby control an average of the magnitude, or the loudness, of an audio output signal.
Quick response is needed in an automatic gain control system if moments of excessive magnitude in the output signal are to be addressed adequately (e.g., a transient such as a door slamming shut), and yet relatively slow gain changes are needed to suppress elevated levels of background noise in the audio output signal. Differing times of termination of gain changes can lead to the introduction of unwanted audio artifacts into the output signal. An AGC that responds both quickly and slowly to the need for gain changes to prevent sudden loudness increases and suppresses background noise can often provide an improved audio output signal for a listener.
The response time of an AGC circuit is commonly characterized by two parameters, the attack and release time. The attack and release times may be defined differently for different industries. For the hearing aid industry, the attack time is the time between the input signal's abrupt increase in level from 55 dBSPL to 90 dBSPL and the point where the output level has stabilized to within 3 dB of the steady value for the 90 dBSPL input sound pressure level (SPL). The release time is the interval between the input signal's abrupt drop from 90 dBSPL to 55 dBSPL and the point where the signal has stabilized to within 4 dB of the steady state value for the 55 dBSPL input sound pressure level (see, e.g., ANSI S3.22-1996, “Specification of Hearing Aid Characteristics”).
As alluded to above, a common problem with AGC circuits employed in hearing aid compression systems is that no single choice of attack and release time adequately compensates for all signals. For instance, a circuit with both a fast attack and release time frequently causes audible “pumping” of the input signal. Conversely, too long a release time will produce audible gaps, especially if the input signal contains short transients resulting in long periods of reduced gain. Attack and release times of 10 ms and 200 ms, respectively, have been used in prior art hearing aids to minimize audible pumping of the input signal. Therefore there is a need in the art for a compression amplifier circuit that adequately addresses the above concerns.